Three-phase Voltage Source Converters (VSCs) generally operate to convert Direct Current (DC) to three-phase Alternating Current (AC), where power can be transferred either from a DC source to an AC grid or load or vice versa. Three-phase VSCs are widely used in adjustable speed motor drives, uninterruptible power supplies, and grid integration of renewable and distributed resources such as solar photovoltaics. For three-phase VSCs using a switching frequency much higher than the fundamental frequency, Space Vector Pulse Width Modulation (SVM) is a popular switching modulation technique. Further, modular three-phase VSCs formed by two or more interleaved, parallel connected VSCs are also becoming increasingly popular due to advantages of reduced voltage or current ratings for the switching devices, reduced current or voltage ripple (hence smaller filter size) by utilizing interleaving, unified design and increased reliability, lower cost and inventory due to standardization, increased redundancy and limp home capability, and distributed thermal load.
One key metric for high performance VSCs is Total Harmonic Distortion (THD). THD is a total distortion in the line current injected from the VSC into the connected load or grid. The main component of THD is the harmonics of the switching frequency (fsw) (i.e., fsw, 2·fsw, etc.). Thus, there is a desire to reduce these harmonics of the switching frequency. However, traditionally, there has been a tradeoff between reducing these harmonics and increasing switching losses. More specifically, traditionally, the harmonics of the switching frequency have been reduced by increasing the switching frequency, which in turn increases switching losses.
Thus, there is a need for a modular three-phase converter having low THD without substantially increasing switching losses.